CAT-3888 or BL22 is an anti-CD22 immunotoxin and in 2015 completed a Phase I clinical (human) trial for the treatment of hairy cell leukemia[1] at the NIH in the U.S. Technically, CAT-3888 is an anti-CD22 immunotoxin fusion protein between a murine anti-CD22 disulfide-linked Fv (dsFv) antibody fragment and an edited copy of bacterial Pseudomonas exotoxin PE38.
The toxin is activated intracellularly, by the low pH of the lysosome into which the entire protein was internalized via the CD22 receptor.
[4] CAT-3888 was succeeded by moxetumomab pasudotox (CAT-8015, HA22), an anti-CD22 immunotoxin comprising a modified Pseudomonas exotoxin and an anti-CD22 antibody fragment.
[5] Like CAT-3888, CAT-8015 changes three amino acids in the antibody fragment to increase the binding affinity for the target molecule.
[citation needed] Research is being carried on directly by Dr. Robert J. Kreitman at the National Cancer Institute.